1. Field of the Invention
The present invention relates generally to a lens assembly for automatic focusing cameras, and more particularly, it relates to a lens mount or lensbarrel of a camera in which an automatic focusing mode and a manual focusing mode can be changed over from one mode to the other.
2. Related Background Art
Some of conventional lens assemblies of the above kind have a construction as shown in FIG. 10 wherein a focusing operation is effected as follows: Namely, driving power from a motor in a camera body is transmitted to a power transmission shaft 52 through a plurality of gears and a coupler 52a and then is transmitted, through a gear 52b, to a cam ring 53 rotatable together with a focusing operation ring 54; and the rotation of the cam ring 53 causes a focusing lens group 51 to shift in the direction of an optical axis, thereby performing the focusing operation. Further, in the above conventional lens assembly, since a manual focusing handling ring 56 is so constructed that it is rotated together with the focusing operation ring, in an automatic focusing mode, the focusing operation ring 54 and the manual focusing handling ring 56 are rotated as a unit. Therefore, in the above conventional lens assembly, there is a problem in that, since the manual focusing handling ring 56 is rotated together with the focusing operation ring 54 in the automatic focusing mode, if the manual focusing handling ring 56 is touched or contacted by a finger of a user who holds the lens assembly during the automatic focusing operation, automatic focusing is impeded. In order to solve this problem, the manual focusing handling ring 56 must be so constructed that the handling ring is difficult to be contacted by the finger of the user who holds the lens. However, with such construction, another problem arises in that the manual focusing operability is worsened.
On the other hand, U.S. Pat. No. 4,413,894 discloses a photographic lens assembly having a mechanism for changing over automatic and manual focusing modes, wherein, when the manual focusing mode is desired, a small gear meshed with an external gear (which is formed on the focusing operation ring and which is rotatable around the optical axis to shift the lens along the optical axis) is shifted along the optical axis to be engaged by an internal gear rotatable together with the manual focusing handling ring around the optical axis.
However, in the lens assembly disclosed in U.S. Pat. No. 4,413,894, in order to shift the small gear in the axial direction and to easily engage the small gear with the internal gear, it is necessary to provide a relatively large amount of backlash; consequently, there is a problem in that an accurate focusing cannot be achieved since the rotation of the manual focusing handling ring is not correctly transmitted to the external gear. On the other hand, if the small gear and the internal gear closely mesh with each other to minimize the backlash, another problem arises in that the changing-over operation from the automatic focusing mode to the manual focusing mode cannot be easily and quickly performed since it is difficult to shift the small gear in the axial direction and to engage the small gear with the internal gear.
Furthermore, the above-mentioned U.S. Pat. No. 4,413,894 also discloses an example wherein a clutch mechanism is provided between the focusing operation ring having the external gear meshed with the small gear and the manual focusing handling ring, thereby directly rotating the focusing operation ring by means of the manual focusing handling ring. The clutch mechanism comprises a grooved wheel having a plurality of radial recesses and provided on one end of the focusing operation ring, and a key protruding from a circumferential inner surface of the manual focusing handling ring and capable of engaging with the recesses The key can be inserted into one of the recesses by shifting the manual focusing handling ring along the optical axis. However, even in this case, similar to the above-mentioned case where the focusing modes are changed over by shifting the small gear in the axial direction, if there is no relatively large amount of slack (or play) between the key and the recess, it will be difficult to easily and quickly change over from the automatic focusing mode to the manual focusing mode; whereas, if there is a relatively large amount of slack, it will be difficult to perform the manual focusing accurately and smoothly.